Blast Hole Pattern Calculator

Estimate burden, spacing, subdrill, and stemming for a surface mine or quarry blast. Enter hole diameter, rock type, bench height, and pattern layout — returns a practical first-pass geometry plus hole density.

Medium rock K

Subdrill factor

0.3×B

Stemming factor

0.7×B

Rectangular S/B

1.15

Units Switches inputs and outputs

Hole diameter D (in) Drill-bit diameter of the blast hole

Rock type K multiplier reflects rock competency

Bench height H (ft) Vertical face height to the floor grade

Pattern layout Spacing multiplier applied to burden

Burden B

14.06ft

Medium rock, rectangular

Spacing S

16.17ft

Subdrill (below grade)

4.22ft

Stemming depth

9.84ft

Holes per acre

191.06

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Tip Empirical rules give a stable starting point. Always tune burden and spacing against bench observations, geology, jointing, and explosive energy before committing to a production pattern.

Worked example

D = 6.75 in, Medium rock (K = 25), rectangular pattern (S/B = 1.15), 40 ft bench.

1. Burden B = K × D / 12 B = 25 × 6.75 / 12 B = 14.06 ft 2. Spacing S = B × 1.15 = 14.06 × 1.15 S = 16.17 ft 3. Subdrill and stemming Subdrill = 0.3 × 14.06 = 4.22 ft Stemming = 0.7 × 14.06 = 9.84 ft 4. Hole density Area/hole = 14.06 × 16.17 ≈ 227.95 sf Holes per acre = 43,560 / 227.95 ≈ 191

Rock type to burden multiplier

Classification	Examples	K	Density
Soft	Sandstone, weak limestone, weathered sedimentary rock	30	140–160 lb/ft³ (2.24–2.56 t/m³)
Medium	Shale, competent limestone, general granite	25	150–170 lb/ft³ (2.40–2.72 t/m³)
Hard	Basalt, quartzite, hard granite	22	165–185 lb/ft³ (2.64–2.96 t/m³)
Very hard	Iron ore, taconite, very dense competent ore rock	18	180–210 lb/ft³ (2.88–3.36 t/m³)

Common mistakes

Using one K across a bench. Rock competency can shift row to row; weathered top benches and competent lower benches often need different multipliers.
Skipping subdrill. Without subdrill the toe almost always stays high, forcing secondary breakage and cleanup.
Over-stemming the collar. More stemming reduces flyrock but cuts the explosive column — past about 0.7×B it starts leaving oversize at the top.
Ignoring the free face. Burden is toward the nearest free face, not the nearest hole. Confirm orientation before loading.

Warn This calculator is a planning estimate. Final blast design must include timing, explosive selection, vibration limits, flyrock control, structure mapping, and local regulatory approval.

FAQ

What is burden in blasting?

Burden is the distance from a blast hole to the nearest free face. It represents the thickness of rock the hole must break in the forward direction. If burden is too large, the face may not pull cleanly; if it is too small, the shot can vent early and throw rock excessively.

Why is spacing different from burden?

Burden governs breakage toward the free face, while spacing governs interaction between adjacent holes in the same row. Spacing is often set equal to or slightly greater than burden depending on whether the pattern is square, rectangular, or staggered.

What is subdrill and why do you need it?

Subdrill is the extra drilled depth below the final floor elevation. It helps the explosive energy break the toe area fully so the bench comes down to grade instead of leaving a hard ridge at the bottom that would require secondary breakage.

How does rock type affect the pattern?

Harder or more massive rock usually needs a smaller burden multiplier, which tightens the pattern. Softer, more fractured, or easier-breaking rock can often be blasted with a wider burden and spacing for the same hole diameter.

What is stemming in a blast hole?

Stemming is the inert crushed rock or similar material placed at the top of a loaded hole. It acts like a plug, trapping explosive gases in the hole long enough to transfer more energy into the bench and reducing early venting, airblast, and flyrock risk.

Sources

MSHA Surface mine blasting guidance ISEE Blasters' Handbook SME Mining Engineering Handbook NIOSH Mining research — blast design

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